This invention relates generally to an automatic on/off circuit for controlling the power supply to electrical equipment. More particularly, this invention relates to an automotive on/off circuit designed to automatically interrupt the power supply to electric automotive accessories, such as radios, cassette players, etc., in a timely manner in order to prevent such accessories from draining power from the automobile battery when the engine is not running.
There are a variety of different electrical accessories adapted for use in an automobile, truck or other craft having an accessible electrical power supply. These accessories are designed to receive power for their operation from the battery of the vehicle by being plugged into the cigarette lighter plug commonly provided in autos, trucks, etc. Besides the aforementioned radios and cassette players, other popular electrical accessories adapted for vehicular use include radar warning receivers, alarm systems, electric fans, electric shavers and the like. Typically, an accessory such as a cassette player or radar warning device is stabilized against undesirable movement while the vehicle travels by mounting the accessory upon the dash of the vehicle or, for example in the case of an electric fan, upon an interior mirror. Commonly, an electrical unit that regularly remains in place and in service within a vehicle is left plugged into the cigarette lighter plug, especially if the vehicle owner is a non-smoker. This practice creates the potential for an unwelcome energy drain upon the vehicle battery because in most automobiles and many other vehicles, the electrical power of the battery, typically twelve volts, is applied constantly to the cigarette lighter plug whether the vehicle's engine is running or turned off. Therefore, the operator must remember to manually turn off the electrical accessory whenever turning off the vehicle engine. Otherwise, the electrical unit plugged into the cigarette lighter plug will stay turned on, thereby causing the vehicle battery to run down and eventually lose its change.
Consequently, a need arose for automatic on/off circuits designed to turn off power to an electrical accessory irregardless of whether or not the operator remembers to manually turn off the accessory. One prior type of approach towards providing such an automatic on/off circuit is exemplified by U.S. Pat. No. 4,733,100, wherein execution of an electrical shutoff is tied to the elapse of a predetermined time period. This type of approach can readily be seen as less than ideal because it is preferable that power flow from the battery be shutoff instantaneously (i.e. simultaneously with the engine shutoff), rather than after expiration of a time period, in order to avoid any unnecessary power drain upon the battery. Moreover, prior approaches disadvantageously monitor whether or not the engine is running in a less than foolproof manner, thereby creating the possibility that the prior on/off circuit may inadvertently elect to bypass a necessary shutoff and continue to allow a power flow to an electrical accessory based on an inaccurate monitoring of the state of the engine. For example, the monitoring approach employed in prior U.S. Pat. No. 4,733,100 utilizes noise sensing apparatus to detect engine ignition noise that signifies a running engine. When the engine is off, the noise sensor detects the absence of engine noise and, after a predetermined time delay, effects a power shutoff. To further clarify this prior approach, it must be noted that the engine "noise" being monitored is not the audible roar of an energized motor, but rather is electrical "noise", specifically the detectable varying electrical noise signal (i.e. ignition noise) that is superimposed upon a nonvarying electrical input signal from the battery when the engine is on. Such an arrangement can give rise to problems because other sources of electrical noise in the automobile electrical system exist, especially if electrical units such as cassette players, buzzers, etc. are tied into the electrical system. Disadvantageously, these other sources may output electrical "noise" which may be detected by "noise" sensors employed in prior approaches, thereby confusing the prior monitoring process into detecting that the engine is on (because of the presence of electrical noise) when actually the engine is off, the battery is being drained, and the detected electrical "noise" is emanating from an alternate source, such as a radio left on at an inaudible volume. This nightmare scenario could result in a dead battery.
From the foregoing, it will be apparent that a significant need exists for an improved automatic on/off circuit adapted to shut off the power flow from a vehicle battery in a manner preventing any unnecessary discharge of the battery's energy. Additionally, an improved automatic on/off circuit is needed which effects instantaneous power shutoff (i.e. deactivation of the electrical system simultaneously with engine shutoff) rather than time delayed deactivation. Moreover, such an improved automatic on/off circuit should monitor the on/off state of the vehicle engine in a manner wherein the improved circuit provides accurate monitoring by eliminating the potential for anything other than the engine to be detected as evidence of a running motor. The present invention fulfills these needs in a relatively inexpensive fashion and provides further related advantages.